1. Field of the Invention
The present invention relates to a semiconductor laser device with a spot-size converter which can couple light to an optical fiber or light waveguide with a high level of efficiency, and to a method of fabricating the semiconductor laser device.
2. Description of the Related Art
Multimedia technologies which have been rapidly developing are likely to enable high-speed, high capacity optical communications (the data transfer rate of which may be 100 Mbps or more) at home as well as at the office in the near future. Among the technologies, the Fiber-To-The-Home (FTTH) is a promising technology which extends an optical fiber from the trunk line to home. In this technology, the output light of a semiconductor laser is required to be introduced into an optical fiber. However, a typical semiconductor laser has its output light of a spot size (about 1 μm) that is largely different from a spot size of single-mode optical fiber (about 10 μm). For this reason, when the semiconductor laser is directly connected with the optical fiber, a great insertion loss is generated due to mode mismatch.
The small spot-size of the semiconductor laser gives rise to a problem that a very small displacement of the spot leads to a great increase in the insertion loss. For example, an about 1 μm displacement between the semiconductor laser and the optical fiber may generate as much as a 10 dB excess loss. To solve this problem, a semiconductor laser with a spot-size converter is considered in which a light waveguide having a larger spot-size than that of a semiconductor laser is integrated along with the semiconductor onto the same substrate.
One method for achieving such a device is a butt junction as shown in FIG. 7A. FIG. 7A shows a semiconductor laser device with a spot-size converter having an ideal structure thereof. In FIG. 7A, a refractive Index coupling-type distributed-feedback semiconductor laser (DFB laser) 200 formed on a semiconductor substrate 100 has a portion thereof removed vertically by etching. In the removed portion, a light waveguide 300 is formed in which a light waveguide layer 301 is sandwiched between light confinement layers 302 and 303. Light output from the semiconductor laser 200 is directly coupled with the light waveguide 300 and the light is then guided in the light waveguide layer 301.
The semiconductor laser device with a spot-size converter thus constructed has a larger spot-size of output light than that of a semiconductor laser, thereby relieving the effect of a very small displacement which occurs when coupling the light with an optical fiber.
However, the above-described conventional example has the following drawbacks.
(1) The ideal shape as shown in FIG. 7A is not actually obtained when the light waveguide is formed in the vertically etched region. The actual shape is, for example, as shown in FIG. 7B. In FIG. 7B, the light waveguide layer 301 is sloped in the vicinity of the place where the semiconductor laser 200 is coupled with the light waveguide 300. In this region, light is affected by the refractive index distribution of this structure so that the proportion of light which is not coupled with the light waveguide layer increases and the coupling rate is therefore greatly reduced from what is expected according to the ideal shape.
(2) When the beam diameter in the vertical direction of the semiconductor laser 200 is not equal to the beam diameter of the inherent mode in the vertical direction of the light waveguide 300, the proportion of light output from the semiconductor laser which is coupled with the light waveguide decreases. The greater the difference between the beam diameters, the more the decrease in the proportion of coupled light.
The above problems (1) and (2) will be described in greater detail below.
FIG. 7D illustrates a concrete example where an InGaAsP-based 1.3 μm-band distributed-feedback (DFB) semiconductor laser is vertically etched and then InGaAsP materials are grown by Metal Organic Chemical Vapor Deposition (MOCVD). The growth rate largely depends on the orientation of the growing plane. A plane having a low growth rate is exposed during the growth, resulting in a shape as shown in FIG. 7B. In this case, a layer structure tilted from a horizontal direction emerges. Therefore, part of the light is affected by the shape and thus reflected or refracted on the interface. The affected part of the light is not coupled with the light waveguide layer 301 and radiated outside the waveguide. In other words, a radiation loss is generated. According to results of experiments conducted by the inventors and the like, it was confirmed that about 1 dB light is radiated by this effect. When the growth was conducted under other conditions different from the above-described conditions, the shape was varied in various ways. Nevertheless, it was impossible to achieve the ideal shape as shown in FIG. 7A and radiation losses in the range of about 0.5 to 1 dB were observed.
Moreover, in this example, the beam diameter in the vertical direction of the semiconductor laser 200 was about 1 μm while the light waveguide layer 301 of the light waveguide 300 was fabricated in such a way as to have a thickness of about 2 μm. This difference resulted in great mode mismatch in coupling light, causing a radiation loss of 1.7 dB to be observed. The sum of both the losses was about 2.7 dB, which requires the semiconductor laser 200 to output light greater than what is actually needed. This increases power consumption by the semiconductor laser 200. In addition, the reliability is reduced. These are big problems. Considering the case where the semiconductor laser is coupled with an optical fiber having a typical mode diameter of about 10 μm, the thickness of the light waveguide layer is preferably greater. In this case, the above-described radiation lose is further increased.